25 August 2006
Epithelial Tissues
Dr. Robert Crissman, Ph.D.

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Levels of Organization

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• Cells – basic unit of all animals
• Tissues – clumps of similar cells grouped together to perform a specific function
• Organs – two or more tissues grouped together to perform a specific function, i.e. kidney
• Systems – two or more organs grouped together to perform a specific function, i.e. urinary system

Basic Tissue Types

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• Epithelial
  • Layer of cells that cover the body’s surfaces. Selective barrier to aid or prevent materials from transferring the surface they cover.
• Muscle
  • A group of cells that change shape, specialized for contraction. Responsible for locomotion, constriction, pumping and peristalsis
• Nerve
  • A group of cells characterized by their excitability. Organism can react to stimuli
  • Two types: neurons and neuroglial cells.
• Connective Tissue
  • A group of cells that connect other tissues together in the body, structurally and metabolically.

Common Structure of Epithelium

• Predominately cellular, making a contiguous sheet of cells forming a layer or membrane
• Have numerous intercellular junctions to hold together to a single, continuous layer
• Polar cells – divide cells into two different regions: basolateral (bottom) and apical (surface) portions
  • All epithelial cells have a free surface
  • Sits on a basement membrane
• Avascular – no blood vessels
  • Blood supply from connective tissue

Embryological Origin

• Epithelia derived from all three germ layers, though most of epithelia are derived from ectoderm and endoderm
  • Ectoderm gives rise to epidermis
  • Endoderm gives rise to the linings of the G.I. tract, respiratory tract, and distal parts of the urogenital tract.
  • Mesoderm gives rise to the linings of internal body cavities
    • Mesothelium – lines the pericardial, pleural and peritoneal cavities
    • Endothelium – lines the blood and lymphatic vessels and heart

Functions of Epithelia

• Functions as selectively permeable barriers, aiding or prevents materials from transversing the surface they cover
• Protection from mechanical, dehydration, and chemical damage
• Secretion – produce and secrete materials into space they are bound
  • Form glands which are part of epithelium basic tissue type
• Absorption – G.I. tract
• Transport – endothelium, lung
• Acts as a sensory surface
• Regenerate and self-repair

Classification of Epithelium by Structure

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• There are two parts of name based on structural charactereistics:
  • Number of layers of cells
    • Unilaminar or simple – single layer
    • Multilaminar or stratified – more than one layer
  • Shape of cell
    • Squamous – thin flattened cells
    • Cuboidal – cells are cubical
    • Columnar – tall narrow cells

Examples of Simple Epithelium Tissue

• Simple squamous epithelium
  • Endothelium and mesothelium
• Simple Cuboidal epithelium
  • Ducts of glands, organ coverings, kidney tubules
• Simple Columnar epithelium
  • Parts of the G.I. respiratory and reproductive tracts
• Pseduostratified columnar epithelium
  • Appears stratified but is not; nuclei located at different levels but all cells sit on basement membrane
  • Some cells are tall and some are short with nuclei at different levels
  • Most of respiratory tract, trachea, epididymis, nasal cavity

Examples of Stratified Epithelium Tissue

• Stratified tissue shapes described by shape of cell on surface
• Stratified squamous epithelium
  • Keratinized – containing fibrous protein called keratin
    • Builds up in cells and kills cells – no nucleus in surface layers
    • Epidermis of skin
  • Non-keratinized – Same appearance as keratinized but have nuclei at surface
    • Wet inner surfaces of oral cavity, esophagus, vagina, conjunctiva of eye
• Stratified squamous mucosal epithelium
  • Why stratified? Because there is a high rate of cell loss and is necessary to have multiple layers
• Stratified cuboidal epithelium
  • Two layers of cuboidal cells
• Transitional epithelium
  • Looks like in transition between stratified cuboidal and stratified squamous
  • Lines most of urinary tract – uroepithelium
  • Undergoes considerable stretching
    • Non-distended uroepithelium – many layers of cells (5-10); large dome shaped cells on surface (cuboidal)
    • Distended uroepithelium – only a few layers (2-3); stretched layers almost like squamous

Specialized Epithelium

• Specialized Epithelium – named on specialized functions
  • Sensory epithelium
    • Gustatory – taste
      • Covers tongue and contains taste buds
    • Olfactory – smell
      • Covers part of nasal passage and contains neuroepithelial cells
    • Stato-acustic – hearing
      • Covers part of inner ear and sense sound and proprioception
  • Germinal epithelium
    • Lines seminiferous tubules of testis – produces germ cells
    • Stratified but shapes are difficult to determine

Polarity of Epithelium

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• Epithelial cells have different domains separated by junctional complex

Apical Domain

• Apical domain
  • Free luminal surface and contains membrane proteins such as carbohydrates forming glycocalyx important for recognition and adhesion
  • Capable of transport
  • Membrane specializations

Microvilli

• Microvilli – small finger-like projections; non-motile; increase cell surface area
  • 1-2 μm, appear in 100’s or 1000’s on cells
  • Together, they form the brush border or striated border
  • Usually large amounts of fluid is being absorbed
  • Internal support system of actin filaments (25-30 filaments)
    • Villin – protein at tip that joins filaments
    • Fimbrin – protein cross-links actin filaments
  • Actin filaments embedded in terminal web at base of microvillus

Stereocillia

• Large finger-like projections; non-motile; increase cell surface area
  • Same internal structure as microvilli but larger
  • Found in epididymis

Cilia

• Cilia – finger-like projections; motile; moves fluids on surface
  • Larger than microfillia
  • Core consists of microtubles called axoneme arranged in 9+2 pattern
  • 2 singlets with 9 peripheral doubles consisting of subunits B and A with 2 arms of dynein proteins
  • Attached to basal body by 9 triplets – basal body is thought to produce microtubules of axoneme
  • Dynein ATPase provides energy that causes arms to transiently bind to the B subunit, sliding the tip to cause bending
  • When Dynein releases subunit B, cilium snaps back upright
  • Beating of cilia are coordinated within one cell and with adjacent cells so all cilia beat in one direction
  • When cilia becomes immotile, can result in primary ciliary dyskenesis, immotile cilia syndrome, Kartagener’s syndrome
    • Structurally lacking Dynein arms and can’t clear respiratory tract of mucus, causing respiratory infections, pneumonia
    • Frequently infertile (eggs can’t get transferred through fallopian tube, resulting in atopic pregnancies) and have malrotation of the heart

Flagella

• Flagella – largest single projection; motile; moves cells
  • Similar to cilia only larger
  • Tail of sperm

Basolateral Domain

• Basolateral domain
• Consist of lateral sides and base of cell
• Biochemically different than apical domain
• Characterized by junctional complex (a.k.a. Terminal Bar) and gap junctions
  • Junctional complex located near the top of the cell where adjacent cells attach to each other and runs circumference of cell
    • Three different junctions: zonula occludens, zonula adherens, macula adherens

Zonula Occludens

• Zonula occludens (a.k.a. tight junctions)
• Most superficial to apical surface
• Fusion of membranes via transmembrane proteins claudins and occluding
  • Creates impermeable barrier – materials cannot pass between cells and have to get through epithelium cell
• Extends around entire circumference of cell
  • Tight (i.e. blood brain barrier) vs. leaky (i.e. capillary), depending on function
• Marks division of apical and basolateral membranes
  • prevents movement of membrane proteins from one domain to another

Zonula Adherens

• Zonula adherens (a.k.a. Adhesion belt)
• Intercellular gap between membranes, 15-20nm
  • Cadherins – transmembrane adhesion molecule or linker protein, extending across gap to hold membrane together
  • Bind to cytoskeleton – actin filaments form dense aggegates on cytoplasmic side linker proteins (vinculin and α-actinin) bind cadherins to actin
• Function: mechanical attachment of adjacent cells

Macula Adherens

• Macula Adherens (a.k.a. Desmosomes)
• “spot weld”
• Gap between membranes, 30 nm
• Dense attachemt plaques on cytoplasmic side consist of desmoplakin and plakoglobin
• Transmembrane proteisn, two cadherins, desmoglein and desmocolin, extend across gap to attach to each other
• Intermediate filament attached to attachment plaques
• Function: mechanical attachment of adjacent cells
  • Clinical relevance: Phemphigus vulgaris
    • Autoimmune disease – antibodies produced against cadherin and destroy desmoglein
    • Patients desmosomes are destroyed in skin, lose integrity of epithelial, causing severe blistering
    • Infections are life threatening

Gap Junctions

• Junction found in the lateral domain
• Functions in cell-to-cell communication
• Memrabne separated by a narrow gap 2-3 nm thick
  • Disc of numerous pores or channels across membranes
    • Each pore is called a connexon, about 1.5 nm in diameter
    • Connexons are structured like a barrel of 6 subunits of connexin
  • Only small ions, small amino acids, cAMP, nucleotides and some vitamins can pass through
  • Larger molecules like proteins, nucleic acids and polysaccharides cannot pass
    • Some are signaling molecules distributed during cell migration and coordination of cilia beating
  • Point of low electrical resistance
    • Can pass along electrical impulse for contraction of smooth muscle to create a peristaltic wave in gut wall

Basal Domain

• Plasma membrane enfolding - tessellation
  • Increases surface area for increased passage of fluids
• Basal lamina – dense layer located outside cell just below basal membrane
  • Hold epithelium to connective tissue
• Hemidesmosomes
  • Appears as half a desmosome on cytoplasmic side
  • Integrin is a transmembrane protein that binds cell membrane to basal lamina

Glands

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• Included as epithelium because they all form as an outgrowth of epithelium
• Classified as exocrine versus endocrine glands
  • If connection to surface epithelium remains as a duct, then the gland is exocrine
    • Secretions go to the surface of the epithelium
  • If the connection to the surface epithelium is lost and does not have a duct, then the gland is an endocrine gland
    • Secretions go directly to the connective tissue and into the blood vessels
• Glands are also classified by structure/morphology, mode of secretion, and type of secretion

Structural Classification

• First structural characteristic is unicellular versus multicellular
  • Unicellular glands – consist of single secretory cells embedded in epithelial sheet
  • Multicellular glands – consists of numberous cells and can be divided intoa duct and a secretory portion

Classification by Gland

• Glands named according to the (1) branching of the duct and (2) the shape of the secretory portion
• Duct can be simple or compound if it branches
• Secretory unit can be tubular or alveolar/acinar (and sometimes both)
  • Tubular glands can be very long and coiled
  • Compound tubuloalveolar – salivary glands
• Surrounded by connective tissue capsule, subdivided by septa to form lobes and lobules

Classification by Mode of Secertion

• Classified by mode of secretion
  • Can be holocrine, merocrine, apocrine, and cytocrine
    • Holocrine – disintegrating cell and contents is secreted; cell is dead
      • i.e. sebaceous glands
    • Merocrine – secretion is through vesicle fuses with membrane and extrudes contents without cytoplasmic loss
      • Most common form of secretion
    • Apocrine – Pinched off portion of the cell is secreted
      • Lipid portion of milk of mammary glands
    • Cytocrine – secrete a whole living cell
      • i.e. ovaries and testis

Classification by Secretion

• Classification by class of secretion
  • Mucus – thick viscous secretion containing mucinogens
    • Slippery, lubricant
    • Cells stain lightly with foamy cytoplasm due to extracted mucus
    • i.e. globlet cells, sublingual salivary gland
  • Serous – Watery secretion, rich in ions or enzymes
    • Acidophilic (red stain) due to presence of secretory machinery
    • i.e. pancreas or sweat glands
  • Mixed Glands – combination of serous and mucus secretory glands
    • Contains serous demilunes, cresent or half-moon on outside of mucous alveolus
  • Sebum – oily secretion, rich in lipids
    • Acts as lubricant
    • i.e. sebaceous glands, meibomian glands of eyelids
  • Ceruminous – waxy secretion
    • Acts for protection of the ear
    • Ceruminous glands of external auditory canal

Objectives

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